Crude gold from various industrial sources arrives on the market in ingot form. These ingots consist either of the electrolyzed gold recovered from ores and may contain the usual trade elements associated with such ores; or the anodes may be cast from recycled casting wastes such as sprues, trimmings, polishings and rejected castings. The recycled casting materials contain the various alloying metals used in the jewelry trade. Trace elements and alloying metals should be removed before the gold can be properly reused. The refinery gold and/or the recycled gold is melted and cast into the ingot conveniently shaped for anode use. These ingots usually contain up to about 99% gold.
The method for gold refining which is generally used in the United States mints consists in electrolyzing these crude ingots as gold anodes in hot acid solution of 7 to 8% gold chloride and 10% hydrochloric acid. Current densities as high as 110 amp/ft.sup.2 of cathode surface are used. The refined gold (mint grade) is electro-deposited on gold foil or rolled sheet cathodes. The electrolysis cells at the mints and assay offices are constructed of glazed porcelain and/or chemical stoneware. During electrolysis, platinum and palladium, present in fractional parts per million, remain dissolved in the haloacidic electrolyte which also serves to precipitate silver ions as AgCl. To prevent the reduction on the cathode of any AgCl particles in its proximity it has been a practice to surround the anodes with cloth bags. This practice confines the insoluble AgCl away from the cathode. At periodic intervals because of the accumulation of impurities, such as the aforementioned silver, as well as borax silicates and dross, from the anode, the bags are changed and some of the electrolyte is drawn off for purificaiton and replaced by fresh acid and the gold chloride solutions. The gold ions pass through the permeable bags to deposit on the cathode.
Glasstone (Introduction to Electrochemistry, (page 496) suggests that the gold, during electrolysis is in tervalent ionized state in the form of AuCl.sub.4.sup.- ions in the electrolyte solution. The purity of the cathode deposited "mint" gold is about 99%.
The jewelry industry for alloying, plating, casting and similar fabrication procedures, needs purer gold, preferably about 99.95% pure (fine) in order to control the desired physical properties.
It is an object of this invention to provide a method for purifying gold to about 99.95% purity and an electrolyte and apparatus wherein this method can be expeditiously practiced. Included among the ancillary objects of this invention, is the realization of such superfine gold which permits proper control of the physical properties of the gold during its further utilization.